2013 f70 issues

[Skin diver]

Ok this will be hard to explain but I'll try my best. I took the boat out a couple of weeks ago and one of the engines was missing and running very rough. Brought the boat home and checked the plugs. Seemed like one of the plugs went bad so I changed that plug and ran it at home. Engine ran fine. Took the boat out this past week and everything was running fine. Cruising at about 4200 rpm. Then I opened it to wot and it started missing again. Ran rough at idle and was missing a lot at higher rpm's. After running it about 40 minutes at 2400 rpm with it missing I tried opening it up and it started running good. Ran it at wot and no problems. Next day started it at home and it was running a little rough with a little miss from time to time. Not too bad. Now here's the weird part. I took the plug from the third cylinder from the top and put it in any other cylinder it ran really rough. Like that cylinder wasn't firing. But when I put it back to the original cylinder it ran descent. Still a little rough but descent. What could be the problem? Water separators and fuel filters were changed and the other engine runs fine. Both are running from the same fuel tank. Please help. Thanks.

[TownsendsFJR1300]

Change all the plugs. Their cheap enough..

Gap them on the tight side of spec's too (as they wear, the gaps open up).

[rodbolt17]

myself?
do a leakdown test.
then fix the fuel system.

[Skin diver]

Ok thanks. I will try changing all the plugs. Forgot to mention that engines were installed a little over two years ago and have only 85 hours. Could this be a warranty issue? I talked to dealer but he wasn't much help.

[TownsendsFJR1300]

Ok thanks. I will try changing all the plugs. Forgot to mention that engines were installed a little over two years ago and have only 85 hours. Could this be a warranty issue? I talked to dealer but he wasn't much help.

Re warranty, it depends on what the issue turns out to be.

If it's a normal maintenance item NOT done, no it wouldn't be covered..
Spark plugs are a maintenance item (not covered). Fuel system filters, again, that's a maintenance item..

**Most times, as Rodbolt posted, it's crap in the fuel system...


I had similar issues as you on my engine awhile ago. 90% of the time it would start breaking up over 4,200 RPM's and when it did go to full throttle, would break up and drop back down to 4,200. Now my idle was fine un-like yours. I had the original spark plugs in it (that apparently are required to be replaced often and were not-maybe 180 hours total). My local shop said replace the plugs (and they looked fine, gap within spec). New plugs installed, NOTHING done to the fuel system, FIXED my issue. Runs to 6,000 RPM W/O issue now..

As a side note, Ethanol fuel goes bad in as quick as a month if NOT stabilized. It sucks up water and causes havoc to the fuel system. If your fuel was old and un-treated, that can and will cause running issues. IME, run Non-Ethanol fuel with a stabilizer (I like "Gas Shok" or K100) along with Yamaha's "Ring Free" -which helps keep the top end and fuel system clean. Both are not cheap but IME, helps prevent your type of issue/down time/trip to the shop....